Vibratory sensors, such as vibratory densitometers and vibratory viscometers, operate by detecting motion of a vibrating element that vibrates in the presence of a fluid to be characterized. Properties associated with the fluid, such as density, viscosity, temperature and the like, can be determined by processing a vibration signal or signals received from one or more motion transducers associated with the vibrating element. The vibration of the vibrating element is generally affected by the combined mass, stiffness, and damping characteristics of the vibrating element in combination with the fluid.
FIG. 1 shows a prior art vibratory sensor comprising a vibratory element and meter electronics coupled to the vibratory element. The prior art vibratory sensor includes a driver for vibrating the vibratory element and a pickoff that creates a vibration signal in response to the vibration. The vibration signal is a continuous time or analog signal. The meter electronics receives the vibration signal and processes the vibration signal to generate one or more fluid characteristics or fluid measurements. The meter electronics determines both the frequency and the amplitude of the vibration signal. The frequency and amplitude of the vibration signal can be further processed to determine a density of the fluid.
The prior art vibratory sensor provides a drive signal for the driver using a closed-loop circuit. The drive signal is typically based on the received vibration signal. The prior art closed-loop circuit modifies or incorporates the vibration signal or parameters of the vibration signal into the drive signal. For example, the drive signal may be an amplified, modulated, or an otherwise modified version of the received vibration signal. The received vibration signal can therefore comprise a feedback that enables the closed-loop circuit to achieve a target frequency. Using the feedback, the closed-loop circuit incrementally changes the drive frequency and monitors the vibration signal until the target frequency is reached.
The target frequency of the fluid can be correlated with the desired phase difference between the drive signal and the vibration signal. Fluid properties, such as the viscosity and density of the fluid, can be determined from the frequencies where the phase difference between the drive signal and the vibration signal is 135° and 45°. These desired phase differences, denoted as first phase difference ϕ1 and second phase difference ϕ2, can correspond to the half power or 3 dB frequencies. A first target frequency ϕ1 is defined as a frequency where the first phase difference ϕ1 is 135°. The second target frequency ω2 is defined as a frequency where the second phase difference ϕ2 is 45°. Density measurements made at the second target frequency ω2, can be independent of fluid viscosity. Accordingly, density measurements made where the second phase difference ϕ2 is 45° can be more accurate than density measurements made at other phase differences.
The closed-loop approach typically measures the frequency of the vibration signal to determine how much to shift the drive signal frequency to achieve the second phase difference ϕ2. Using the measured frequency, a relationship between the measured frequency and the phase is used to determine if there is a phase difference of 45° between the drive signal and the vibration signal. However, the closed-loop approach to measuring fluid properties has some associated issues. For example, the frequency of the vibration signal must first be measured to obtain a desired phase difference between the vibration signal and the drive signal. This can be problematic because the vibration signal can be very small relative to noise. As a result, measuring the frequency from the vibration signal requires filtering. This filtering can cause delays in the frequency measurement, which can cause instability in drive control algorithms. Additionally, any unfiltered noise in the vibration signal will be reproduced in the drive signal. Noise in the drive signal can cause drive instability as well as inaccuracies in the frequency measurement.
Accordingly, there is a need for a method for generating a drive signal for a vibratory sensor that does not require the frequency measurements associated with the closed-loop approach.